The present invention relates to an echo canceling apparatus that deals separately with changing and unchanging echo components.
U.S. Pat. No. 5,610,909 discloses an echo canceling apparatus of this type. Referring to FIG. 1, in one embodiment, the apparatus comprises an input terminal 200 coupled to a loudspeaker 201, a microphone 202 coupled to an output terminal 203, a long-response echo canceler 204, a delay adjustment generator 205, an adjustable delay unit 206, and a short-response echo canceler 207. An incoming electrical signal received at the input terminal 200 is supplied to the adjustable delay unit 206, long-response echo canceler 204, and loudspeaker 201. The loudspeaker 201 converts the incoming signal to an acoustic signal, part of which is picked up as an echo by the microphone 202. The long-response echo canceler 204 generates a first estimated echo or echo replica signal from the incoming signal, by synthesizing a long impulse response of the echo path from the loudspeaker 201 to the microphone 202, and subtracts the first echo replica signal from the microphone output signal. The adjustable delay unit 206 delays the incoming signal by an amount determined by the delay adjustment generator 205 on the basis of the impulse response synthesized by the long-response echo canceler 204. The short-response echo canceler 207 generates a second echo replica signal from the delayed incoming signal, by synthesizing a short impulse response of the echo path. The short-response echo canceler 207 subtracts the second echo replica signal from the output of the long-response echo canceler 204, and supplies the resulting outgoing signal to the output terminal 203.
Both echo cancelers adapt their impulse responses to changes in the echo path, but the short-response echo canceler 207 adapts more quickly than the long-response echo canceler 204. The echo typically has a relatively long, substantially unchanging first component due to acoustic reflection from stationary surfaces such as walls, and a mostly shorter, rapidly changing second component due to acoustic reflection from a moving person or moving object. The long-response echo canceler 204 accurately cancels the first component, the delay adjustment generator 205 determines the relative position of the second component within the first component, and the short-response echo canceler 207 cancels the second component.
The long and short impulse responses are normally synthesized as tap coefficients of respective adaptive filters. In this context, xe2x80x98longxe2x80x99 and xe2x80x98shortxe2x80x99 mean that the long-response echo canceler 204 has more tap coefficients than the short-response echo canceler 207. The delay adjustment generator 205 operates by, for example, detecting the position of the largest or most variable tap coefficient of the long-response echo canceler 204, and centering the tap coefficients of the short-response echo canceler 207 around that position.
One problem observed in this apparatus is that when it is employed in a small enclosed acoustic environment, such as a small room or the cabin of an automobile, the echo often has little or no rapidly changing component. When the rapidly changing component is absent, nothing is gained from the use of two separate echo cancelers, and there is a significant power-consumption penalty.
A second problem is that a short-response echo canceler is less accurate than a long-response echo canceler. When the rapidly changing echo component is small or absent, the short-response echo canceler 207 may degrade, rather than improve, the quality of the outgoing signal.
A third problem is that while the impulse response synthesized by the short-response echo canceler 207 has a fixed length, the rapidly varying echo component does not. Thus at some times, the second echo replica signal may be too short to cancel all of the rapidly changing echo component. At other times, the second echo replica signal may be too long, and may degrade the outgoing signal by inaccurately attempting to cancel a non-existent echo component.
A fourth problem arises in the double-talk state, when both the near-end and far-end parties speak at once. In this state, the echo cancelers 204, 207 cannot adapt correctly to changes in the echo path. In particular, the short-response echo canceler 207 cannot track the rapidly changing echo component. To worsen matters, during and after the double-talk state, the delay adjustment generator 205 may misjudge the position of the rapidly changing component, completely invalidating the output of the short-response echo canceler 207 and severely degrading the outgoing signal. The resulting harm to signal quality can far exceed the benefit obtained when the delay adjustment generator 205 and short-response echo canceler 207 operate correctly.
One object of the present invention is to cancel a changing echo component of variable length.
Another object of the invention is to reduce power consumption.
Another object is to avoid signal-quality degradation caused by unnecessary echo cancellation.
Another object is to avoid signal-quality degradation caused by erroneous cancellation of a changing echo component in the double-talk state.
The invented echo canceling apparatus has a tapped delay unit that stores a plurality of most recent samples of an incoming signal. A long-response echo canceler generates a first echo replica signal from the stored samples, which are output from the taps of the tapped delay unit, and subtracts the first echo replica signal from an outgoing signal to cancel an echo of the incoming signal, obtaining a residual signal.
According to a first aspect of the invention, the apparatus also has a control unit that calculates an echo cancellation characteristic for each tap of the tapped delay unit, thereby identifying taps corresponding to a changing component of the echo. The number of taps identified in this way is variable and may be zero, as when the echo has no changing component. A switching unit selects the samples output from the taps identified by the control unit. A short-response echo canceler generates a second echo replica signal from the selected samples and subtracts the second echo replica signal from the residual signal.
According to a second aspect of the invention, the short-response echo canceler is replaced by a step-gain generator that generates step gains controlling the rate of adjustment of tap coefficients in the long-response echo canceler. The control unit controls the step-gain generator so as to increase the rate of adjustment of the tap coefficients corresponding to the selected taps.
According to a third aspect of the invention, the control unit detects whether the source of the outgoing signal is silent, and disables the short-response echo canceler when the source is not silent.
In the first and second aspects of the invention, the number of taps supplying samples to the short-response echo canceler, or the number of tap coefficients with high step gains, is adapted to the varying length of the changing component of the echo.
When the echo has no changing component, the short-response echo canceler does not operate, so power is conserved.
These features also prevent degradation of the outgoing signal due to unnecessary echo cancellation by the short-response echo canceler.
The third aspect of the invention prevents degradation of the outgoing signal due to inaccurate operation of the short-response echo canceler in the double-talk state.